1. Field of the Invention
The present invention relates generally to medical methods and apparatus. More particularly, the present invention relates to methods and apparatus used to improve shear loading capacity of a spinal segment. The methods and apparatus disclosed herein may be used alone or in combination with other orthopedic procedures intended to treat patients with spinal disorders such as degenerative spondylolisthesis.
A source of chronic low back pain is degenerative spondylolisthesis (DS). Degenerative Spondylolisthesis is a common clinical condition that typically presents in the 5th to 8th decades. The listhesis, or anterior translation of the superior vertebra relative to the inferior vertebra, is associated with degenerative changes which make the facet joints less resistant to shear forces seen by the segment.
As the center of mass of the human body is almost always in front of the spine, there is typically a net shear force exerted transversely across segments of the lower lumbar spine during activities of daily living. Each motion segment of the spine includes a disc and two facet joints (a left and a right facet joint) which share shear loads. As the facet joints degenerate, their typical coronal orientation becomes more sagittal, particularly in the superior section of the facet joint, further away from the pedicle. The facet joints' resistance to shear decreases as they become more sagittally oriented, and thus a larger share of the shear load is borne by the disc. The typical finding on flexion/extension films in patients with degenerative spondylolisthesis is that the amount of anterior translation increases when the segment is in flexion, and decreases when the segment is in extension. In the extended position, more of the facet joint is engaged, and thus the overall resistance to shear is increased.
Patients with DS typically present with symptoms of stenosis, and these symptoms can be addressed surgically with a decompression/laminectomy and fusion. Unfortunately, however, while decompression relieves pressure from nerves that cause pain, the removal of tissue involved in the decompression can also increase the flexion instability seen in DS, and, over time, the listhesis can increase and cause symptoms to recur. Because of the risk that a stand-alone decompression will increase post-operative instability, the standard of care in the United States is to treat degenerative spondylolisthesis patients with a decompression to address the presenting symptoms and a fusion to prevent progression of the instability and recurrence of symptoms. A surgeon performs decompression to relieve pressure on the nerve roots, typically at L4-L5, L3-L4, L5-S1, or elsewhere along the lumbar region of the spine. Bone is removed as required in order to provide pain relief, while still leaving some pieces of the bony structure intact. Less invasive techniques emphasize resection of only tissues causing neurological injury, and maintaining natural anatomy as much as possible. Often the superior portion of the superior spinous process in the affected spinal segment is left intact along with inferior portion of the inferior spinous process of the spinal segment. Additionally, a significant portion of the lamina will also be left intact.
Sometimes fusion and decompression are performed without instrumentation. Non-instrumented fusions typically require the post-operative use of a lumbar brace for 3-6 months to ensure that the fusion has the best chance to heal. Even with the brace, the non-union rate still can be as high as about 40-50%. Bracing is not particularly effective in limiting segmental motion, is expensive and irritating for patients and is associated with morbidities such as skin pressure sores.
Therefore, fusion often includes instrumentation of the affected spinal segment including the use of pedicle screws and stabilization rods that have high morbidity and complication rates. For example, because shear loading is transferred from the pedicle screws to the stabilization rod at their interface, sites of fatigue result, which can lead to instrumentation failure.
It would therefore be desirable to provide improved devices and methods that are simpler, less invasive and easier to use than existing treatment options that require fusion, rigid instrumentation and decompression procedures. Furthermore, it would be desirable for such improved devices and methods to reduce or eliminate anterior translation of the superior vertebra relative to the inferior vertebra. It would also be desirable for these devices and methods to increase the ability of the spinal segment to bear shear loads. It would also be desirable to have devices and methods able to withstand loading along the spinal segment with lower failure rates than traditional instrumentation. Such methods and devices also should be cost effective and manufacturable. Some or all of these objectives will be met by the devices and methods disclosed herein.
2. Description of the Background Art
U.S. Patent Publication No. 2005/0216017A1 now U.S. Pat. No. 7,458,981 is described below. Other patents and published applications of interest include: U.S. Pat. Nos. 4,966,600; 5,011,494; 5,092,866; 5,116,340; 5,282,863; 5,395,374; 5,415,658; 5,415,661; 5,449,361; 5,456,722; 5,462,542; 5,496,318; 5,540,698; 5,609,634; 5,645,599; 5,725,582; 5,902,305; Re. 36,221; U.S. Pat. Nos. 5,928,232; 5,935,133; 5,964,769; 5,989,256; 6,053,921; 6,312,431; 6,364,883; 6,378,289; 6,391,030; 6,468,309; 6,436,099; 6,451,019; 6,582,433; 6,605,091; 6,626,944; 6,629,975; 6,652,527; 6,652,585; 6,656,185; 6,669,729; 6,682,533; 6,689,140; 6,712,819; 6,689,168; 6,695,852; 6,716,245; 6,761,720; 6,835,205; Published U.S. Patent Application Nos. 2002/0151978; 2004/0024458; 2004/0106995; 2004/0116927; 2004/0117017; 2004/0127989; 2004/0172132; 2005/0033435; 2005/0049708; 2006/0069447; Published PCT Application Nos. WO 01/28442 A1; WO 02/03882 A2; WO 02/051326 A1; WO 02/071960 A1; WO 03/045262 A1; WO 2004/052246 A1; WO 2004/073532 A1; and Published Foreign Application Nos. EP 0322334 A1; and FR 2 681 525 A1.